BEIJING, Oct 19 (Reuters) - China’s industrial output growth accelerated to a three month high in September, while fixed asset investment growth continued to decline, falling to the slowest pace since December 1999.

But the government has also ordered some steel mills and factories in northern areas to cut back or halt production in coming months to reduce choking winter air pollution, which some analysts have said could hit the industrial sector.

Fixed-asset investment expanded 7.5 percent in the first nine months of the year, missing forecasts for 7.7 percent growth, and marking the slowest rate of growth since a 6.3 percent reading in December 1999, according to Reuters calculations.

Investment growth has slowed in recent years amid efforts by authorities to move away from investment-driven economic growth.

But private sector fixed-asset investment continues to lag state spending, slowing to 6.0 percent growth for Jan-Sept, compared to 11.0 percent growth in investment by state firms. Private investment rose 6.4 percent in the previous period.

Retail sales rose 10.3 percent in September on-year, beating expectations and indicating consumption continues to hold up well. Retail sales growth has hovered in the 10 to 11 percent range for the last two years.

Analysts had forecasted sales would rise 10.2 percent, slightly more than in August.

After a surprisingly strong start to the year, the world’s second largest economy is expected to easily meet or beat the government’s full-year growth forecast of around 6.5 percent.

But most China watchers expect activity will slow in coming months as higher financing costs and measures to cool the heated property market start to weigh on activity. (Reporting by Kevin Yao; Writing by Elias Glenn; Editing by Sam Holmes)